The price was the highest ever paid for a Rockwell work and more than twice the $20 million estimate, the New York Times reported.
Two other Rockwells, "The Gossips" and "Walking to Church," sold for $8.45 million and $3.2 million, in line with Sotheby's estimates. The total for the three paintings was $57.8 million.
The buyers of all three paintings remained anonymous. The paintings were sold by the three sons of Kenneth J. Stuart, the Post's longtime art director, who had received them as gifts from the artist.
The three paintings had been on loan to the Norman Rockwell Museum in Stockbridge, Mass., for many years.
All three works were painted as covers for the Saturday Evening Post. Rockwell painted 321 Post covers between 1916 and 1963, work that made him the best-known and most popular U.S. artist of his era, although not the most critically esteemed.
"Saying Grace" depicts an older woman and a boy, presumably her grandson, bowing their heads in prayer before eating in a diner. The magazine paid Rockwell $3,500 in 1951, the equivalent of about 10 times that amount now, the Times said.
The cover was a big hit with Post readers. In 1955, a poll found it was ranked the most popular.
The previous high price for a Rockwell painting was $15.4 million for "Breaking Home Ties," which was auctioned at Sotheby's in 2006.